This invention relates generally to a method and apparatus for controlling an angle of a swashplate pivotally attached to a variable displacement hydraulic pump and, more particularly, to a method and apparatus for controlling an angle of a swashplate as a function of a power limit of the pump.
Variable displacement hydraulic pumps, such as axial piston variable displacement pumps, are widely used in hydraulic systems to provide pressurized hydraulic fluid for various applications. For example, hydraulic earthworking and construction machines, e.g., excavators, bulldozers, loaders, and the like, rely heavily on hydraulic systems to operate, and hence often use variable displacement hydraulic pumps to provide the needed pressurized fluid.
These pumps are driven by a constant speed mechanical shaft, for example by an engine, and the discharge flow rate, and hence pressure, is regulated by controlling the angle of a swashplate pivotally mounted to the pump.
Operation of the pumps, however, is subject to variations in pressure and flow output caused by variations in load requirements. It has long been desired to maintain the pressure output of the pumps in a consistent manner so that operation of the hydraulic systems is well behaved and predictable. Therefore, attempts have been made to monitor the pressure output of a pump, and control pump operation accordingly to compensate for changes in loading.
A problem incurred when a pump is operated under varying loads is that the power available to the pump, i.e., from the engine, is limited. Therefore, although certain hydraulic pressure and hydraulic flow rate demands may be made of a pump in operation, it may not be feasible to supply the power required for the desired pressure and flow rate combination. It is desired, therefore, to control the operation of the pump in a manner that is consistent with overall power demands placed on the total hydraulic machine.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention a method for controlling a variable displacement hydraulic pump having a swashplate pivotally attached to the pump is disclosed. The method includes the steps of determining a desired swashplate angle as a function of a power limit of the pump, determining an actual swashplate angle, determining a value of discharge pressure of the pump, moving a servo valve spool to a desired position as a function of the desired swashplate angle, the actual swashplate angle and the discharge pressure, and responsively moving the swashplate to the desired swashplate angle position.
In another aspect of the present invention an apparatus for controlling a variable displacement hydraulic pump is disclosed. The apparatus includes a swashplate pivotally attached to the pump, a control servo operable to control an angle of the swashplate relative to the pump, a servo valve having an output port connected to the control servo and an input port connected to a pump output port, means for determining an actual swashplate angle, means for determining a value of discharge pressure of the pump, and a controller connected to the servo valve and adapted to determine a desired swashplate angle as a function of a power limit of the pump, and to move a servo valve spool in the servo valve to a desired position as a function of the desired swashplate angle, the actual swashplate angle, and the discharge pressure.